Physostigmine, also called esrine, and certain of its derivatives are well known anticholinesterase inhibitors, and are useful in the treatment of glaucoma, Myasthenia Gravis, Alzheimer's disease, and as an antidote against poisoning with organophosphates.
Physostigmine was first isolated in 1864 by Jobst and Hesse after it was originally introduced into England in the form of the Calabar bean in 1840 by Daniell, a British medical officer. During the last century, physostigmine has been used as a treatment for glaucoma, and in the reversal of atropine-induced coma. More recently, physostigmine has been used effectively as an antidote to several drugs possessing central anticholinergic properties.
In the last fifteen years, the knowledge of receptor function has been advanced considerably by studies for the acetylcholine-receptor-ion-channel complex (AChR) of the neuromuscular junction. The occurrence of nicotinic AChRs at very high densities in Torpedo and Electrophorus electric organs made this membrane receptor easily available for study. In addition, specific chemical probes for the different active sites have contributed significantly to understanding of the morphology and function of this receptor.
In the early 1970's, alpha-bungarotoxin (alpha-PGT) was isolated from snake venoms and was found to bind irreversibly and specifically to the acetylcholine (ACh) recognition site on the nicotinic acetylcnoline-receptor-ion-channel complex. The availability of such a highly selective probe permitted the isolation, purification, functional reconstitution into artificial lipid membranes and, ultimately, cloning of the different subunits which comprise the nicotinic acetylcholine-receptor-ion-channel complex.
The pharmacological charactization of another class of toxins, the histrionicotoxins, isolated from the skin secretion of frogs of the family Dendrobatidae, disclosed an important new class of sites on the nicotinic AChR. These sites, which are distinct from the agonist recognition site and are most likely located on the ion channel component of the AChR, are responsible for allosteric alterations or noncompetitive blockage of neuromuscular transmission. Drugs with distinct and well-known pharmacological activities on the peripheral as well as central nervous systems, such as tricyclic antidepressants, phenothiazine antipsychotics, the hallucunogenic agent phencyclidine (PCP), local anesthetics, antimuscarinics, anticholinesterase agents, and may other have been shown to modify noncompetitively the activation of the AChR.
Additionally, microscopic kinetic models are now available for study. More refined biophysical techniques, such as the patch-clamp method which allows the recording of single-channel currents, have disclosed finer aspects of the permeability changes initiated by the binding of the agonist molecules.
On the biochemical front, rapid-mixing methods have been used to measure accurately early conformational transitions of nicotinic receptor molecules. These studies showed that activation of the nicotinic AChR comprises complex microscopic gating kinetics, i.e. the conformational changes of the protein may involve transitions through many states, on different time scales, and with distinct voltage dependencies.
It has been reported that the agonist recognition site at the nicotinic acetylcholine receptor has strong stereospecificity, as revealed by the optical isomers of certain semi-rigid agonists, cf. Spivak et al., Mol. Pharmacol. 23: 337-343, 1983. The ion channel sites, on the other hand seemed not to be stereospecific, as revealed by the similar qualitative and quantitative actions of the enantiomers of perhydrohistrionicotoxin at the nicotinic AChR, cf. Spivak et al., FEBS Lett. 163: 189-193, 1983.
The natural isomer of physostigmine has shown blocking as well as agonist properties at the neuromuscular AChR, whereas (+)-physostigmine has negligible ChE inhibitory activity, as reported by Brossi et al., FEBS Lett. 201: 190-192, 1986. In protection studies in rats, (+)-physostigmine was found most effective as a pretreatment drug against multiple lethal doses of sarin, cf. Albuquerque et al., Fundam. Appl. Toxicol. 5: 182-203, 1985. It appears that direct interactions of the carbamates with the postsynaptic nicotinic AChR may account for their beneficial effects in the observed protection. The effectiveness of these carbamates in protecting against organo-phosphates appears to be directly related to the ability of the former to decrease the hyperactivation caused by the accumulation of the neurotransmitter. The evidence acquired from such studies is of fundamental importance in the assessment of new drugs in the treatment of cholinergic disorders, including myasthenia gravis and Alzheimer's disease. Beneficial results are more likely to be achieved with cholinergic agonists resistant to ChE inhibition and those crossing the blood-brain barrier.